In H.264 and MPEG-4, a video signal is hierarchically split into a sequence, frame, slice, macro block, and block. The block becomes a smallest processing unit. With regard to encoding, residual data of the block is obtained through intra frame prediction or inter frame prediction. Also, the residual data is compressed through transformation, quantization, scanning, run length coding, and entropy coding. As an entropy coding technique, there is context-based adaptive binary arithmetic coding (hereinafter referred to as CABAC). In accordance with CABAC, a context index ctxIdx is used to determine one context model, an occurrence probability of a least probable symbol (LPS) or a most probable symbol (MPS) of the determined context model and information vaIMPS about which binary value between 0 and 1 corresponds to the MPS are determined, and binary arithmetic coding is performed based on vaIMPS and a probability of the LPS.